The present invention relates to a process for copper-plating a wafer which is suitable for forming a copper wiring on a wafer.
Copper has a higher allowable current density than aluminum and other metals and thus causes little power loss in wiring. Thus, copper has been noted as a semiconductor wiring material and has found growing application.
For the formation of copper wiring, a sputtering process, CVD process, an electroless plating process and an electroplating process have been employed. Among these processes, the electroplating process has become the most popular technique. This is because the electroplating process can form a deposit at a high rate to give an increased throughput, and can form a deposit at ordinary temperature and pressure that requires an apparatus which is less expensive and which can be handled more easily than that required for the other processes.
Examples of the anode for use in the electroplating process include a soluble copper electrode, and an insoluble electrode comprising Pt-plated titanium. These anodes individually have disadvantages.
In some detail, the use of the soluble copper electrode has the following disadvantages:
(1) In order to obtain uniform dissolution of copper ion, a black film is formed on the surface of the electrode. However, the film comes off to form a powder which is then dispersed in the plating solution. The powder is then incorporated into the thin copper layer to raise the layer resistivity. A suspended powder removing film on the way of the plating solution pipes is clogged up with the powder.
(2) The distribution of the copper wiring thickness on the wafer must be uniform. However, because the surface of the soluble electrode is dissolved ununiformly in thickness during prolonged use, the distance between the electrodes is ill-balanced, thus disturbing the current distribution and hence producing a variation in the thickness of deposit on the wafer.
(3) The soluble electrode continues to be dipped in the plating solution even during replacement of the wafer to be plated. Thus, copper is dissolved even during suspension of electric power, making it difficult to control the copper concentration.
(4) As the copper plate material constituting the soluble electrode, oxygen-free copper or tough pitch copper is used. However, according to JIS H3100, the purity of these materials is 99.96 at the highest. The amount of impurities contaminating the plating solution is as great as 0.5 g per 1.2 tons of copper deposit, calculated based on the assumption that the plating bath has been energized for 1 KAH (kilo-ampere-hour).
On the other hand, when Pt-plated titanium is used as an insoluble electrode, organic additives such as a lubricant, brightener and surface active agent incorporated in the plating solution are decomposed at a high rate because the Pt-plated titanium anode exhibits a high oxygen generation potential. This causes the additives to be consumed in an increased amount. Further, the copper film-forming properties change with time. Moreover, the cell voltage increases, thus raising the requirements of the power unit.
Where a wafer is plated with copper by a conventional copper-plating process, the insoluble electrode can be used with less difficulty than the foregoing soluble copper electrode. However, as discussed the insoluble electrode has various disadvantages, and there is a need to solve the above mentioned problems of the prior art.
It is therefore an object of the invention to provide a process for copper-plating a wafer using an anode which hardly contaminates the plating solution, causes no disturbance of current distribution due to changes in the shape of the surface of the anode, and causes little oxidative destruction of additives.
The foregoing object of the invention will become apparent from the following detailed description and examples.
In accordance with the invention, the following wafer plating processes are provided to accomplish the foregoing objectives.
(1) A process for copper-plating a wafer which comprises electroplating a wafer with an electrode comprising a corrosion-resistant metal substrate and a coat mainly composed of iridium oxide provided on the substrate as an anode and the wafer as a cathode in a solution containing copper ion.
(2) The copper-plating process according to (1) above, wherein the anode is an insoluble electrode comprising a corrosion-resistant metal substrate and a coat mainly composed of iridium oxide and further containing a metal or metal oxide selected from the group consisting of platinum, tantalum, titanium, niobium and oxides of these metals provided on the substrate.
(3) The copper-plating process according to (1) or (2) above, wherein a neutral membrane or ion exchange membrane is interposed between the anode and the cathode as a separating membrane.
The foregoing insoluble electrode comprising a corrosion-resistant metal substrate and a coat mainly composed of iridium oxide provided on the substrate exhibits an anode potential low enough to prevent the organic components of additives contained in the plating solution from undergoing oxidative destruction during the generation of oxygen and excellent corrosion resistance even under such plating conditions.
The present invention will be further described hereinafter.
The substrate having a coat mainly composed of iridium oxide may be a corrosion-resistant metal such as a valve metal (e.g., Ti, Ta, Nb).
The desired electrode can be formed by providing a coat of iridium oxide alone or a mixture or solid solution thereof with an oxide of one or more metals belonging to the platinum or non-platinum group on the foregoing corrosion-resistant substrate. The process for preparing the desired electrode is not specifically limited. Various processes involving thermal decomposition or the like as disclosed in JP-B-46-21884 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent applicationxe2x80x9d) and JP-B-46-3954 may be employed.
The desired electrode can also be formed by applying a mixture or solid solution of iridium oxide with one or more other non-platinum group metals to the corrosion-resistant substrate. The foregoing non-platinum group metal is preferably Ti, Ta or Nb.
The foregoing metal oxide constituting the coat may be a stoichiometric metal oxide as well as a nonstoichiometric metal oxide or oxide having lattice defects.
The preferred range for the amounts of each of metal oxide is as follows.
Ir: 1 to 60 g-metal oxide/m2 
Ta: 1 to 50 g-metal oxide/m2 
Pt: 1 to 100 g-metal/m2 
Ni: 1 to 30 g-metal oxide/m2 
Nb: 1 to 40 g-metal oxide/m2 
Further, a valve metal layer or a single valve metal oxide or mixed oxide of valve metals may be interposed between the corrosion-resistant substrate and the foregoing oxide layer to enhance the corrosion resistance of the electrode.
The foregoing coat is a catalyst layer having a thickness of from about 1 to 10 xcexcm. In this arrangement, even when used as an electroplating anode, only the catalyst is liable to be consumed. Thus, the electrode according to the invention shows no change in external dimension during the course of an electroplating operation. This makes it possible to maintain a uniform current distribution over an extended period of time and hence form a wiring deposit on the wafer to a stabilized thickness.
The foregoing insoluble electrode can be applied to a plating bath having a structure without particular limitation such as a horizontal or vertical electroplating bath.
The amount of the anode mainly composed of iridium oxide that is consumed when energized during plating is about 5 mg/KAH or less, which is two orders of magnitude smaller than the concentration of impurities from the soluble electrode (about 500 mg/KAH). Thus, the amount of contaminants from the anode can be considerably reduced.
Further, the electrode according to the invention exhibits an oxygen generation potential of about 500 mV lower than that of a Pt-plated electrode, demonstrating that it has a low capacity of oxidatively destroying the organic components of additives. Further, the low oxygen generation potential also results in the lowering of plating voltage during plating, making it possible to reduce the electric power cost as well. In accordance with the copper plating process of the invention, a neutral membrane or an ion exchange membrane can be interposed between the anode and the cathode as a separating membrane. This reduces the amount of organic components such as animal fats and oils incorporated into the copper plating solution and that are oxidatively destroyed on the surface of the anode. Hence, use of such a membrane can stabilize the concentration of additives in the vicinity of the cathode.
The foregoing neutral membrane may be properly selected from polyethylene membranes, polyester membranes and polypropylene membranes commercially available from Yuasa Corp. and Nakao Filter Media Co., Ltd. depending on the type of the plating bath to which it is applied.
The foregoing ion exchange membrane may be properly selected depending on the intended application. In particular, Nafion 117 (produced by Du Pont Inc.) is preferred.